Continuously variable power split transmission (CVPST) system for use in a hybrid vehicle

ABSTRACT

The present invention is concerned with a continuously variable power split transmission(CVPST) system for use in a hybrid vehicle. The vehicle has a first motor, such as an internal combustion (IC) engine and a second motor, such as an electric motor. The system has a step-up gearbox for providing additional fixed speed ratios to extend overall transmission conversion range (CR) of the system. The system also has a speed variator and a countershaft being operatively connected to the second motor of the vehicle so that any power input applied by the second motor to the countershaft reduces the power flowing through the speed variator. Also, an adjustment of the variator speed ratio i v  allows a passage from underdrive i UD  to overdrive i OD  range of speed ratios in the step-up gearbox to be synchronized.

FIELD OF THE INVENTION

[0001] The present invention is concerned with a continuously variable power split transmission system (CVPST) for use in a hybrid vehicle, in particular in a vehicle having a first motor power supply and a second motor power supply.

BACKGROUND OF THE INVENTION

[0002] Conventional continuously variable transmission (CVT) systems are well known. CVT applications rely on shaft-to-shaft power transmission in their operation. The problem with CVT systems is that, at high-speed modes of operation, the pulley-belt action limits the capacity of the systems, as the belt tends to jitter due to slippage. Furthermore, CVT losses are greatest at low speeds and at overdrive, and reduced to a minimum when the velocity ratio i_(v) is about 1.00.

[0003] To solve this problem which is quite important in automotive applications, it has already been suggested to use a continuously variable power split transmission (CVPST) system such as in U.S. Pat. No. 5,167,591 by COWAN. Such a system is actually a variable pulley system (or “variator”) coupling two of the three rotating elements of a planetary gear train. A main feature of such a system is that it carries only a fraction of the total power flowing through the variator, thus increasing the power envelope of the potential engine application. This feature also reduces the power losses associated with power transmission, especially at the low-speed-high-torque modes, while providing continuously variable transmission ratio capability. Thus, as it can be understood, CVPST systems accomplish two important functions, which are desirable in any automotive application.

[0004] First of all, at low speeds, only a fraction of the power flows through the variator thereby increasing the power envelope of the application while reducing the losses by the same factor.

[0005] Secondly, the system provides a “stepless” transmission ratio variation. This, combined with appropriate engine throttle control (motorized throttle), allows for optimum engine performance and minimum fuel consumption and emissions.

SUMMARY OF THE INVENTION

[0006] It has now been discovered, and this is the object of the present invention, that if one supplies power from, for example, an electric motor to the control element (countershaft) of a CVPST in an automotive vehicle, one may further reduce the power flowing through the variator. This power or excess of torque can be used entirely to overcome the acceleration resistance power of the vehicle, so that even during the transient conditions such as acceleration, the working point of the internal combustion (IC) engine is kept all the time on the Optimum Efficiency Curve.

[0007] Thus, use of a CVPST in a hybrid vehicle comprising an IC engine and an electric motor connected to the countershaft of the CVPST allows for further power split and therefore an increase of efficiency and power envelope.

[0008] Use of only electric power for acceleration allows the working point of the IC engine to stay all the time on the Optimum Efficiency Curve (OEC), even during the transient conditions of the acceleration process. Incremental and simultaneous adjustment of the opening of the throttle (rack position in Diesels) and the ratio of the CVPST can achieve this result. Due to these adjustments, while vehicle accelerates with the excess of torque from electric motor (supplied through the countershaft), the intersection point of the Opening of the Throttle (OT) curve of the IC engine and Driving Resistance Curve (DRC) of the vehicle, can follow the Optimum Efficiency/Emission Curve (OEC).

[0009] In other words, further unloading of the variator belt is possible with use of electric power (torque) applied to the countershaft of the CVPST. Advantageously, a start-up clutch, or first clutch, may be provided on the countershaft to allow the variator to rotate with the engine, even when the clutch is disengaged and vehicle is not moving. This prevents the transmission to be stuck in the high gear (stalling of the engine during a panic braking on the slippery surfaces). After restarting of the engine, the variator turns, and it is possible to bring it back to low gear ratio position.

[0010] In use, the CVPST can be connected to the transmission of the vehicle as is disclosed in U.S. Pat. No. 5,167,591. However, to avoid negative circulation power, use will preferably be made of an idler gear to ensure that the direction of rotation of both the sun and the ring gear, be the same. This is important in order to achieve effective power split. It should be noted that many other configurations would be possible by connecting the variable element to any two of the three rotating elements, with and without idler gears, to change the direction of rotation. The configuration shown here is however the easier one that can be directly related to the Automotive CVPST application.

[0011] Therefore, according to the present invention, there is provided a continuously variable power split transmission (CVPST) system for use in a hybrid vehicle, the vehicle having a first motor power supply and a second motor power supply, the system comprising:

[0012] (a) a step-up gearbox having an input shaft and an output shaft, the output shaft being connected to a main ratio gear set, the gearbox providing additional fixed underdrive i_(UD) and overdrive i_(OD) speed ratios to extend overall transmission conversion range (CR) of the system; and

[0013] (b) an infinitely adjustable, power split variable speed transmission unit comprising:

[0014] a sun shaft operatively connected to the first motor power supply of the vehicle;

[0015] a planetary gear set having a sun gear fixed to the sun shaft, at least two planet gears meshed with the sun gear, a ring gear meshed with the planet gears, a control gear or control sprocket coupled to the ring gear, a planet carrier having planet axes for mounting the planet gears, the planet carrier being keyed to the input shaft of the gearbox;

[0016] a countershaft extending parallel to the sun shaft, the countershaft being operatively connected to the ring gear through a first clutch and an additional control gear with idler gear or an additional control sprocket with chain so that the countershaft and ring gear rotate in the same direction, the countershaft being also operatively connected to the second motor power supply of the vehicle through a second clutch; and

[0017] a speed variator mounted between the sun shaft and the countershaft to adjustably vary the speed of the countershaft and thereby adjusting the speed and torque of the planet carrier through an adjustment of a variator speed ratio i_(v);

[0018] whereby, on the one hand, any power input applied by the first motor power supply to the sun shaft is split into two streams merging on the planet carrier and input shaft of the gearbox, one passing through the speed variator, countershaft and ring gear, the other directly through the sun gear, and, on the other hand, any power input applied by the second motor power supply to the countershaft reduces the power flowing through the speed variator, and whereby, the adjustment of the variator speed ratio i_(v) allows a passage from the underdrive i_(UD) to overdrive i_(OD) fixed speed ratios in the step-up gearbox to be synchronized (and vice versa).

[0019] According to the invention, there is also provided a continuously variable power split transmission (CVPST) system for use in a hybrid vehicle, the vehicle having a first motor power supply and a second motor power supply, the system comprising:

[0020] (a) a step-up gearbox having an input shaft and an output shaft, the output shaft being connected to a main ratio gear set, the gearbox providing additional fixed underdrive i_(UD) and overdrive i_(OD) speed ratios to extend overall transmission conversion range (CR) of the system; and

[0021] (b) an infinitely adjustable, power split variable speed transmission unit comprising:

[0022] a sun shaft operatively connected to the first motor power supply of the vehicle;

[0023] a planetary gear set having a sun gear fixed to the sun shaft, at least two planet gears meshed with the sun gear, a ring gear meshed with the planet gears, a control gear or control sprocket fixed to the ring gear, a planet carrier having planet axes for mounting the planet gears, the planet carrier being keyed to the input shaft of the gearbox;

[0024] a countershaft extending parallel to the sun shaft, the countershaft being operatively connected to the ring gear through a first clutch and an additional ratio gear with idler gear or an additional control sprocket with chain so that the countershaft and ring gear rotate in the same direction, the first clutch being mounted on the countershaft, the first clutch coupling the countershaft with the additional control gear or sprocket, the additional control gear or sprocket being freely mounted on the countershaft, the countershaft being also operatively connected to the second motor power supply of the vehicle through a second clutch; and

[0025] a speed variator mounted between the sun shaft and the countershaft to adjustably vary the speed of the countershaft and thereby adjusting the speed and torque of the planet carrier through an adjustment of a variator speed ratio i_(v);

[0026] whereby, on the one hand, any power input applied by the first motor power supply to the sun shaft is split into two streams merging on the planet carrier and input shaft of the gearbox, one passing through the speed variator, countershaft and ring gear, the other directly through the sun gear, and, on the other hand, any power input applied by the second motor power supply to the countershaft reduces the power flowing through the speed variator, and whereby, the adjustment of the variator speed ratio i_(v) allows a passage from the underdrive i_(UD) to overdrive i_(OD) fixed speed ratios in the step-up gearbox to be synchronized (and vice versa).

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The invention as well as its numerous advantages will be better understood by the following non restrictive description of preferred embodiments made in reference to the appending drawings in which:

[0028]FIG. 1 is a perspective view of main elements of a CVPST system with an in-line step-up gearbox, according to the present invention;

[0029]FIG. 2 is a side view of main elements of a CVPST system with an in-line step-up gearbox and a dry multi-disc clutch on a countershaft, according to the present invention;

[0030]FIG. 3 is a diagram of the ratio of CVPST and power split as a function of the variator speed ratio (i_(v));

[0031]FIG. 4 is a schematic diagram illustrating power branching when an electric power supply P_(in)(E) is connected to a countershaft (CS) of a CVPST system, according to the present invention;

[0032]FIGS. 5A and 5B are diagrams showing the power of an IC engine (P_(in)(IC)), the power through a variator (P_(v)) and the ratio of power P_(in)(E)/P_(in)(IC) as a function the power P_(in)(E) when an electric motor power supply is used, for variator speed ratios i_(v)=0.5 and 2.0, according to the present invention;

[0033] FIGS. 6 to 9 are diagrams demonstrating the influence of coefficient ξ=P_(in)(E)/P_(in)(IC) for 0.00, 0.50, 1.00, 1.30 respectively on additional power split resulting from use of electric power applied to the countershaft;

[0034]FIG. 10 is a diagram of an engine performance map demonstrating acceleration of a hybrid vehicle with power from IC engine and an electric motor, according to the present invention;

[0035]FIG. 11 is a diagram showing Driving Resistance Curves (DRC) shown on the Engine Performance Map (EPM) of an ultra light car; and

[0036]FIG. 12 is diagram showing synchronization curves and speeds of the output shaft at first and second stages.

DETAILED DESCRIPTION OF THE FIGURES

[0037] Referring to FIGS. 1 and 2, there is shown a continuously variable power split transmission (CVPST) system 1 that can be used in a hybrid vehicle having a first motor power supply and a second motor power supply, according to the present invention. For example, the first motor power supply can be an internal combustion (IC) engine and the second motor power supply can be an independent electric motor. However, the first and second motor power supplies may be any types of motors. Furthermore, the second motor power supply may be powered via the first power supply through a differential system.

[0038] The CVPST system 1 includes a step-up gearbox 3 which is preferably a conventional two-speed and reverse in-line step-up gearbox having a primary input shaft 5 and a secondary output shaft 7 with first stage ratio gears 9, second stage ratio gears 11, a synchronizer ring 13 and reverse ratio gears 15. As can be seen, some of the gears may have a helical shape for increased performance as is known in the art. Alternatively, a parallel step-up gearbox can also be used instead of an in-line step-up gearbox, to reduce the length of transmission.

[0039] The output shaft 7 of the gearbox 3 is connected to a main ratio gear set which is preferably formed of a first gear 14 fixed on the output shaft 7 and a second bigger gear 16 connected to a differential housing (not shown). The differential housing transmits the output power (P_(out)) to the wheels of the vehicle.

[0040] The purpose of the step-up gearbox 3 is to provide additional fixed speed ratios to extend the overall transmission conversion range (CR) of the CVPST system 1.

[0041] As shown in FIGS. 1 and 2, the main input power (P_(in)(IC)) supplied to the CVPST system 1 is provided through a motor shaft 17 that is preferably connected to a internal combustion (IC) engine, or first motor.

[0042] The infinitely adjustable, power split variable speed transmission unit used in the CVPST system 1 comprises a sun shaft 19 that is operatively connected to the first motor power supply of the vehicle. In the example shown in FIGS. 1 and 2, the first motor power supply or IC engine is connected to the motor shaft 17 which is keyed on the sun shaft 19.

[0043] The infinitely adjustable, power split variable speed transmission unit also comprises a standard planetary gear set 21 having a sun gear 23 fixed to the sun shaft 19. The planetary gear set 21 also comprises two or more planet gears 25 meshed with the sun gear 23, a ring gear 27 meshed with the planet gears 25, a control gear or control sprocket 29 coupled to the ring gear 27, and a planet carrier or spider 31 having planet axes 33 for mounting the planet gears 25. The planet carrier 31 is keyed to the input shaft 5 of the gearbox 3.

[0044] The infinitely adjustable, power split variable speed transmission unit also comprises a countershaft 35 extending parallel to the sun shaft 19. The countershaft 35 is operatively connected to the ring gear 27 through a first clutch 37 and an additional control gear with idler (not shown) or an additional control sprocket 41 with silent chain 39, so that the countershaft 35 and ring gear 27 rotate in the same direction. It will be understood by those skilled in the art, that a belt or any other means may be used instead of the chain 39 with control sprockets 29, 41 to achieve the same results.

[0045] The countershaft 35 is also operatively connected to the second motor power supply of the vehicle, which is preferably an electric motor (P_(in)(E)), through a second clutch (not shown). As mentioned above, the second motor power supply can be any appropriate motor such as a combustion engine, or can even be derived from the first motor power supply through a differential system. However, the latter would not increase the total output power (P_(out)), it would only decrease the power going through the variator.

[0046] Preferably, the control gear or control sprocket 29 is fixed on or integral with the ring gear 27 and the first clutch 37 is mounted on the countershaft 35. The first clutch 37 couples the countershaft 35 with the additional control gear or additional sprocket 41, which is freely mounted on the countershaft 35.

[0047] Preferably, the sun shaft 19, which is keyed to the motor shaft 17, is coaxially journalled within the planet carrier 31 and the motor shaft 17 is adapted to extend through the input shaft 5 of the gearbox 3.

[0048] The infinitely adjustable, power split variable speed transmission unit further comprises a standard speed variator 45 mounted between the sun shaft 19 and the countershaft 35 to adjustably vary the speed of the countershaft 35. Thereby, the speed and torque of the planet carrier 31 is adjusted through an adjustment of a variator speed ratio i_(v).

[0049] In the embodiment shown in the accompanying drawings, the speed variator 45 preferably comprises a pair of pulleys 47 and 49 respectively mounted on the sun shaft 19 and countershaft 35 in such a manner as to extend in a same plane perpendicular to the sun shaft and countershaft axes. The pulleys 47 and 49 consist of half-V sheaves on which an endless V-belt 51 is frictionally engaged. Each of the pulleys 47 and 49 has a fixed sheave and a movable sheave for controllably adjusting the distance between the sheaves. This particular arrangement which is known per se allows the belt 51 to change its active radius of contact with the sheaves and thereby vary the rotational speeds of the pulleys with respect to each other, in other words changing the speed ratio of the variator 45, typically from i_(v =)0.5 to i_(v)=2.0. Other mechanisms, such as hydraulic pistons, known per se may also be used to move the half-V sheaves to achieve the same result.

[0050] Preferably, the first clutch 37 can be a dry multi-disc clutch activated through a hydraulic piston 43, which allows the variator 45 to run always with the engine, even if the first clutch 37 is disengaged. That prevents the CVPST system 1 to get stuck in the high gear ratio in the event of blocking wheels and stalling engine.

[0051] Alternatively, the first clutch 37 may be a multi-disk wet clutch mounted between the ring gear 27 and control gear or control sprocket 29 of the planetary gear set 21.

[0052] The following section describes the power split with only an IC engine input (P_(in)(IC)). The velocity ratio R for the CVPST system 1 considering that the input comes through the sun shaft 19 and the output through the planet carrier 31 is given by: $\begin{matrix} {{\frac{\omega_{in}}{\omega_{out}} = {\frac{i_{v}\left( {1 + i_{g}} \right)}{{i_{v}i_{g}} + i_{c}} = R}}{{Where}\text{:}}\begin{matrix} {i_{g} = {\frac{N_{S}}{N_{R}}\quad {{Sun}/{Ring}}}} \\ {i_{c} = {\frac{N_{o}}{N_{c}}\quad {Control}}} \\ {i_{v} = {\frac{r_{o}}{r_{i}}\quad {Variator}}} \end{matrix}} & (1) \end{matrix}$

[0053] The velocity ratio i_(cvpst) can also be expressed in a different way using Z_(p)/Z_(s) ratio which characterizes the planetary gear set 21, the control gear ratio i_(c), variator ratio i_(v), (which is changing in this case from 0.5 to 2.0) and by i_(s−up) (i_(UD) or i_(OV)), step-up gearbox 3, is given by: $\begin{matrix} {{i_{cvpst} = {\frac{\omega_{in}}{\omega_{out}} = {\frac{1}{C*\left( {1 + \frac{K}{i_{v}}} \right)}*i_{s - {{up}{({{UD}\quad {or}\quad {OD}})}}}}}}\text{where:}} & (2) \\ {C = {\frac{1}{\left\lbrack {2*\left( {1 + \frac{Z_{p}}{Z_{s}}} \right)} \right\rbrack}\quad \left( {{planetary}\quad {set}\quad {constant}} \right)}} & (3) \\ {K = {\frac{1 + {2\frac{Z_{p}}{Z_{s}}}}{\pm i_{c}}\quad \left( {{CVPST}\quad {constant}} \right)}} & (4) \end{matrix}$

[0054] Where:

[0055] Z_(p)=number of teeth on planet gear

[0056] Z_(s)=number of teeth on sun gear

[0057] i_(c)=control gear (sprocket) ratio

[0058] The presence of the chain 39 or idler gear (power split) or the absence of it (power recirculation), changes the sign of the control gear ratio i_(c) from positive to negative. In the power split case, the velocity ratio is positive. In the power recirculation case, the velocity ratio is negative, giving rise to the so-called “geared neutral” condition. This condition represents the singularity of zero denominator in equations (1), (2), and (5) which in turn corresponds to the point of motion reversal for the planet carrier 31. This feature, although seemingly desirable in some cases, may produce a serious overload on the variator 45. This is certainly not desired for automotive applications. This is basically for two reasons: first, the efficiency is low as power is being recirculated through the variator 45, and second, too high ratio of the “first (or low) gear i_(UD)” is not recommended due to negative influence of rotational elements (masses) of the powertrain, including the engine.

[0059] As can be seen from the equation (5) expressing power going through the variator 45, the split of power in the CVPST is a function of the variable i_(v). Value K is constant for a given transmission and can take positive or negative value, depending on the sign of i_(c) (control gear ratio). $\begin{matrix} {P_{v} = {P_{in}\frac{K}{i_{v} + K}}} & (5) \end{matrix}$

[0060] Referring to FIG. 3, it can be seen that for the big numerical values of i_(v)=2.0 (Under Drive), the power branch going through the variator 45 represents approximately 57% of P_(in), the remaining portion of power goes directly through the sun gear 23 of planetary gear set 21. It can be seen, that the biggest power split occurs at variator ratio i_(v)=2.0 (underdrive or low gear), which is useful during the acceleration of the vehicle from stand still. For i_(v)=0.5 (overdrive), the power transmitted through the belt 51 of the variator 45 is 84% of P_(in).

[0061] Referring now to FIGS. 1, 2, and 4, the following section describes the power split using both an IC engine and an electric motor input. In particular, FIG. 4 shows a schematic diagram of power branching when the electric motor power supply (P_(in)(E)) is connected to the countershaft 35 (CS) and the IC engine power supply (P_(in)(IC)) is a connected to the sun shaft 19.

[0062] In this case, the CVPST system 1 has a control element through which the portion of power is circulating, see equation (5). If the electric power is supplied to the countershaft 35 as shown on FIG. 4, the power flowing through the variator 45 is lower due to the fact that the force on a planet gear 25 meshing with ring gear 27 must be the same as the force on the same planet gear 25 meshing with the sun gear 23. As it will be demonstrated below, and with reference to FIGS. 5A, 5B, 6, 7, 8 and 9, the power passing through the variator 45 can be reduced to zero, or even reversed for different values of coefficient ξ.

[0063] Assuming that mechanical efficiency of the system is 100%, η=1.0. We can derive the following equations for power balance:

P _(out) =P _(in(IC)) +P _(in(E))  (6)

[0064] and also: $\begin{matrix} {P_{{in}{(E)}} = {{\xi*P_{{in}{({IC})}}\quad {or}\quad \xi} = \frac{P_{{in}{(E)}}}{P_{{in}{({IC})}}}}} & (7) \end{matrix}$

[0065] Where:

[0066] P_(out)=power out from spider

[0067] P_(in(IC))=power supplied to sun—shaft from IC engine

[0068] P_(in(E))=electric power supplied to countershaft

[0069] ξ=coefficient expressing ratio of $\frac{P_{{in}{(E)}}}{P_{{in}{({IC})}}}$

[0070] Using the equation (5) for power passing through the variator 45, we can derive: $\begin{matrix} {P_{v} = {{P_{{in}{({IC})}}\left( {1 - \xi} \right)}\quad \frac{K}{i_{v} + K}}} & (8) \end{matrix}$

[0071] If ξ=0 in equation (8) and referring to FIG. 6, no electric power is supplied and only IC engine power supplied to the sun shaft 19 (for this particular case, K=2.667, I_(v)=2.0) is flowing as follows: 57% of P_(in(IC)), through variator 45 and 43% of P_(in(IC)), through the gears. If the electric power amounts for half of IC engine power, ξ=0.5, see FIG. 7, the power flowing through variator 45 will be only 28.5% of P_(in(IC)), and the power flowing through the gears (sun gear 19) will represent the remaining 71.5% of P_(in(IC)). The explanation of this shift of power to the gears, which constitutes a very positive phenomenon, derives from the necessity of the balance of forces on the planet gears 25, as explained above.

[0072] Referring to FIGS. 5A, 5B and 8, at the value ξ=1.0, the power transmitted through the variator 45 is zero. This happens when the power from electric motor supplied to countershaft 35 is equal to the power supplied by IC engine to the sun shaft 19 (20 kW in this case). If the value ξ is >1.0, see FIG. 5A, 5B and 9, the excess of electric motor power will start to flow from the countershaft 35 to the sun shaft 19 (negative P_(v) values on the diagrams). This is a significant shift of power from variator 45 to gears. It will allow for the use of much smaller variators (belts, or any other friction elements), or for the application of CVPST to higher torque engines, where so far, the conventional CVT has a hard time to compete with manual or automatic transmissions.

[0073] As can be seen from FIGS. 5A and 5B, influence of power split is much more pronounced for variator ratio i_(v)=2.0 which prolongs the effective belt life, or would allow for use of the same capacity belt in applications with bigger engines.

[0074] As mentioned above, the function of the step-up gearbox 3 is only to provide additional fixed speed ratios to extend overall transmission conversion range (CR) of the system. The conversion range for the present example is given by the following formula: $\begin{matrix} {{{CR} = \frac{i_{{cvpst}{({i_{v} = {2\quad 0}})}}*i_{UD}}{i_{{cvpst}{({i_{v} = {0\quad 5}})}}*i_{OD}}}\text{where:}\begin{matrix} {{{i_{{cvpst}{({i_{v} = {2\quad 0}})}}{is}\quad {cvpst}\quad {ratio}\quad {at}\quad i_{v}} = 2.0};} \\ {{{i_{{cvpst}{({i_{v} = {0\quad 5}})}}\quad {is}\quad {cvpst}\quad {ratio}\quad {at}\quad i_{v}} = 0.5};} \\ {{i_{UD} = {{fixed}\quad {underdrive}\quad {gearbox}\quad {ratio}}};} \\ {i_{OD} = {{fixed}\quad {overdrive}\quad {gearbox}\quad {{ratio}.}}} \end{matrix}} & (9) \end{matrix}$

[0075] Referring to FIG. 10, an Engine Performance Map (EPM) has been used to illustrate the acceleration of a vehicle where a CVPST is assisted by an electric motor. Such an EPM has shown that proper simultaneous changes in CVPST ratio (R) and opening of the throttle (OT) permits to achieve an efficient and least polluting acceleration of the vehicle. The acceleration of the vehicle assisted by the electric motor allows the IC engine to work at its optimum conditions, even during transient conditions as long as all excess torque for acceleration comes from electric motor.

[0076] It is a very well known fact that during the acceleration of the conventional vehicle (with only IC engine), the working point of the engine can not be kept on Optimum Efficiency Curve (OEC), FIG. 10. Even if during a constant speed movement of the vehicle the engine working point is located on OEC, the additional opening of the throttle is needed to create an excess of torque for acceleration. The engine working point shifts into the area of much higher values of BSFC (Brake Specific Fuel Consumption). New Driving Resistance Curve (DRC) is involved (passing through point 1′), which this time, includes the acceleration resistance of the vehicle at the same transmission ratio (R1).

[0077] As the vehicle starts to accelerate under (lets assume) constant torque (segment 1-1′) supplied to the countershaft CVPST, or the output shaft, the opening of the throttle and ratio of the CVPST is incrementally and simultaneously adjusted, so that the working point of the IC engine can stay on the OEC. Opening of the throttle changes incrementally from OT1 position to OT2, and CVPST ratio will also be incrementally adjusted from R1 through R3, and back (in direction of lower ratios) to R2.

[0078] Once the IC engine achieves RPM2, throttle position is OT2 and transmission ratio is R2, the new IC engine working point is point 2. At this point the electric power supply is cut off in order to stabilize the new IC engine working point on new Constant Power Curve (CPC2) which intersects the curve OT2 and DRC corresponding to R2. As can be seen from the diagram (FIG. 10) point 2 is also located on the Optimum Efficiency Curve.

[0079] This is exactly the expected result. Even during the transient state of IC engine working conditions it is possible to keep the engine working point in the optimum range of BSFC, on OEC. To achieve that however, the Electronic Throttle Control (or Motorized Throttle) is needed, together with automatic adjustment of the transmission ratios. A microprocessor is much better equipped to perform this task than is the driver. Acceleration with CVT-s, if the transmissions are properly controlled, is superior to that with manual transmissions. Although manual step ratio transmissions have slightly superior efficiency, they have a limited possibility to fully explore the Engine Performance Map field.

[0080]FIG. 11 shows engine flywheel torque in function of engine RPM for an ultralight car. Also shown are: Wide-Open Throttle (WOT) engine torque curve, the Constant Power Curves (CPC) and Driving Resistance Curves (DRC) for two stages of CVPST. It can be seen that the DRC of maximum speed of the car can be achieved on the underdrive (UD) stage with, or on overdive (OD) stage with I_(v)˜1.85, (close to 2.00) where the power transmitted through the gears is highest, with also better efficiency.

[0081] Referring to FIG. 12, to pass from one stage to another, when synchronizer ring 13 keyed to output shaft of 7 the step-up gearbox 3 (SUGB) is in neutral position (between two gears mounted freely on the output shaft) the speed of the first motor is maintained at the same level as before disconnect (throttle opening diminished). In this moment, the speed of the gear on the output shaft 7 of SUGB 3 to which the synchronizer ring 13 is to be connected, is synchronized with that of the synchronizer ring 13 (and therefore output shaft 7 of SUGB 3) to allow for trouble (and noise) free connection of synchronizer ring internal teeth with the external connecting teeth on the free wheeling output shaft gear 9 or 15. Once the new connection is established, the power (opening of the throttle) to the first motor is re-established. The total transmission ratio is the same as before the switch of stages, and will almost be not perceptible to the driver. The synchronizer is used only to avoid unpleasant noise of connecting stationary synchronizer ring 13 (on output shaft 7) to rotating gears, when vehicle is not rolling. A friction cone ring in the synchronizer slows down the rotating gear, before the connection between the two is made. When the vehicle is running, the synchronization is achieved by the use of the variator 45 which achieves much better action.

[0082] Hence, during the switch of ratio in step-up gearbox in passing from underdrive to overdrive stage, the momentary power disruption is necessary to keep the engine RPM at a disconnect value and during this time the rotational speeds of the gears to be connected are synchronized by change of the variator ratio i_(v).

[0083] The synchronizer ring 13 in the step-up gearbox 3 is used only to prevent noise during the engagement of the transmission into drive mode, especially in cold start conditions.

[0084] Although preferred embodiments of the present invention have been described in detail herein and illustrated in the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments and that various changes and modifications may be effected therein without departing from the scope or spirit of the present invention. 

What is claimed is:
 1. A continuously variable power split transmission (CVPST) system for use in a hybrid vehicle, the vehicle having a first motor power supply and a second motor power supply, the system comprising: (a) a step-up gearbox having an input shaft and an output shaft, the output shaft being connected to a main ratio gear set, the gearbox providing additional fixed underdrive i_(UD) and overdrive i_(OD) speed ratios to extend overall transmission conversion range (CR) of the system; and (b) an infinitely adjustable, power split variable speed transmission unit comprising: a sun shaft operatively connected to the first motor power supply of the vehicle; a planetary gear set having a sun gear fixed to the sun shaft, at least two planet gears meshed with the sun gear, a ring gear meshed with the planet gears, a control gear or control sprocket coupled to the ring gear, a planet carrier having planet axes for mounting the planet gears, the planet carrier being keyed to the input shaft of the gearbox; a countershaft extending parallel to the sun shaft, the countershaft being operatively connected to the ring gear through a first clutch and an additional control gear with idler gear or an additional control sprocket with chain so that the countershaft and ring gear rotate in the same direction, the countershaft being also operatively connected to the second motor power supply of the vehicle through a second clutch; and a speed variator mounted between the sun shaft and the countershaft to adjustably vary the speed of the countershaft and thereby adjusting the speed and torque of the planet carrier through an adjustment of a variator speed ratio i_(v); whereby, on the one hand, any power input applied by the first motor power supply to the sun shaft is split into two streams merging on the planet carrier and input shaft of the gearbox, one passing through the speed variator, countershaft and ring gear, the other directly through the sun gear, and, on the other hand, any power input applied by the second motor power supply to the countershaft reduces the power flowing through the speed variator, and whereby, the adjustment of the variator speed ratio i_(v) allows a passage from the underdrive i_(UD) to overdrive i_(OD) fixed speed ratios in the step-up gearbox to be synchronized.
 2. The system according to claim 1, wherein the control gear or control sprocket of the planetary gear set is fixed on or integral with the ring gear and the first clutch is mounted on the countershaft, the first clutch coupling the countershaft with the additional control gear or additional sprocket being freely mounted on the countershaft.
 3. The system according to claim 1, wherein the step-up gearbox is an in-line or parallel step-up gearbox.
 4. The system according to claim 1, wherein the step-up gearbox comprises a synchronizer ring.
 5. The system according to claim 1, wherein the first clutch is a dry or wet multi-disc clutch.
 6. The system according to claim 1, wherein the sun shaft is coaxially journalled within the planet carrier, the sun shaft being keyed to a motor shaft, the motor shaft being operatively connected to the first motor power supply, the motor shaft extending through the input shaft of the gearbox.
 7. The system according to claim 1, wherein the speed variator comprises: a pair of pulleys respectively mounted onto the sun shaft and the countershaft in such a manner as to extend in the same plane, each of the pulleys having a fixed sheave and a movable sheave; an endless V-belt mounted onto the pulleys; and means for adjusting a distance between the fixed and movable sheave of each pulley.
 8. The system according to claim 1, wherein the ratio of the radius of the planet gears to the radius of the sun gear is ranging between 1.8 and 2.5.
 9. The system according to claim 1, wherein the main gear ratio set comprises a first gear fixed on the output shaft and a second gear connected to a differential housing.
 10. A continuously variable power split transmission (CVPST) system for use in a hybrid vehicle, the vehicle having a first motor power supply and a second motor power supply, the system comprising: (a) a step-up gearbox having an input shaft and an output shaft, the output shaft being connected to a main ratio gear set, the gearbox providing additional fixed underdrive i_(UD) and overdrive i_(OD) speed ratios to extend overall transmission conversion range (CR) of the system; and (b) an infinitely adjustable, power split variable speed transmission unit comprising: a sun shaft operatively connected to the first motor power supply of the vehicle; a planetary gear set having a sun gear fixed to the sun shaft, at least two planet gears meshed with the sun gear, a ring gear meshed with the planet gears, a control gear or control sprocket fixed to the ring gear, a planet carrier having planet axes for mounting the planet gears, the planet carrier being keyed to the input shaft of the gearbox; a countershaft extending parallel to the sun shaft, the countershaft being operatively connected to the ring gear through a first clutch and an additional ratio gear with idler gear or an additional control sprocket with chain so that the countershaft and ring gear rotate in the same direction, the first clutch being mounted on the countershaft, the first clutch coupling the countershaft with the additional control gear or sprocket, the additional control gear or sprocket being freely mounted on the countershaft, the countershaft being also operatively connected to the second motor power supply of the vehicle through a second clutch; and a speed variator mounted between the sun shaft and the countershaft to adjustably vary the speed of the countershaft and thereby adjusting the speed and torque of the planet carrier through an adjustment of a variator speed ratio i_(v); whereby, on the one hand, any power input applied by the first motor power supply to the sun shaft is split into two streams merging on the planet carrier and input shaft of the gearbox, one passing through the speed variator, countershaft and ring gear, the other directly through the sun gear, and, on the other hand, any power input applied by the second motor power supply to the countershaft reduces the power flowing through the speed variator, and whereby, the adjustment of the variator speed ratio i_(v) allows a passage from the underdrive i_(UD) to overdrive i_(OD) fixed speed ratios in the step-up gearbox to be synchronized.
 11. The system according to claim 10, wherein the step-up gearbox is an in-line or parallel step-up gearbox.
 12. The system according to claim 10, wherein the sun shaft is coaxially journalled within the planet carrier, the sun shaft being keyed to a motor shaft, the motor shaft being operatively connected to the first motor power supply, the motor shaft extending through the input shaft of the gearbox. 